In a honeycomb structure broadly used in a catalyst carrier for purifying exhaust gas, a higher purifying performance has been demanded in order to cope with exhaust gas regulations which have been tightened year by year. On the other hand, there has also been a demand for reduction of pressure losses from requirements such as low fuel consumption and high output.
Under these situations, the movement for the improvement of purification performance becomes enhanced more and more by reducing the pressure loss with increasing the open pore area at the cell opening end face of the honeycomb structure, and activating a catalyst at an early stage after starting of engine with reducing thermal capacity of the partition walls by making the thickness of partition walls of a honeycomb structure thinner and thinner.
On the other hand, with development of reduction of the wall of the honeycomb structure, erosion phenomenon has raised a new problem, in which various foreign matters mixed in an exhaust gas collide against the partition wall existing in the cell opening end portion of the honeycomb structure, and the partition wall breaks.
Moreover, to solve the problem, a honeycomb structure has been proposed comprising a partition wall strengthened portion whose strength is raised as compared with another partition wall portion in the partition wall already existing in the opening end portion (Japanese Patent Application Laid-Open No. 2000-51710), and various methods for disposing the partition wall strengthened portions have also been proposed.
As the method for disposing the partition wall strengthened portion, a method has been known in which a substrate of the honeycomb structure mainly composed of a cordierite-forming material is fired, and thereafter a slurry containing the cordierite-forming material dispersed in a dispersion medium is coated to the partition wall existing in the cell opening end portion of the substrate, dried, and fired (the same publication as the above, etc.).
However, in this method, a firing step requiring a long time is necessary in two stages: the firing of the substrate; and the firing for disposing the partition wall strengthened portion, and a large problem remains to be solved with regard to production efficiency, manufacturing cost and the like.
To solve the problem, a method has been suggested in which the slurry containing a partition wall strengthening material dispersed in the dispersion medium is coated to the partition wall existing in the cell opening end portion, thereafter dried, and fired, so that the firing of the substrate and the forming of the partition wall strengthened portion are performed in one firing stage (the same publication as the above).
However, in this method, at present, any concrete study has not been made on the difference of material composition as to the substrate before firing and after firing (see the same publication as the above). Especially, an organic binder or the like added for a purpose of raising the strength of the partition wall is usually present in the substrate before firing, but any consideration has not been taken in a point that the organic binder is mostly a water-soluble compound such as methyl cellulose.
Therefore, a partition wall strengthened portion forming step that has heretofore been performed after the firing is performed using the slurry containing the partition wall strengthening material dispersed in water as such before the firing. Then, the organic binder is dissolved and swollen, the partition wall of the obtained honeycomb structure is deformed, and the structure cannot tolerate practical use because of a drop of isostatic strength and the like in the present situation.
On the other hand, by the use of the slurry containing a partition wall strengthening material such as talc dispersed in a non-water-soluble dispersion medium such as kerosene, the problem of the deformation of the partition wall is solved, but the partition wall strengthening material including talc has bad dispersibility, and easily sediments and aggregates. Therefore, it has been difficult to stably coat a certain amount of the partition wall strengthening material to the partition wall. Therefore, this manufacturing method has had a problem that strength, porosity and the like of the formed partition wall strengthened portion easily become non-uniform, and a honeycomb structure having desired resistance to erosion cannot be stably obtained.
The present invention has been developed in consideration of the above-described problem of the conventional technique, and an object thereof is to provide a manufacturing method capable of largely improving enhancement of productivity and reduction of product costs while stably manufacturing a honeycomb structure having a desired isostatic strength and resistance to erosion.